1. Field of the Invention
The present invention relates, in general, to a biaxially stretched polyester film excellent surface properties and, more particularly, to a biaxially stretched polyester film superior in slip property and abrasion resistance.
2. Description of the Prior Art
By virtue of the superior intrinsic, physical and chemical properties of polyester which polyethylene terephthalate is representative of, polyester is, in general, used to being formed into a film which is used for various purposes, for example, a base film for a magnetic tape, a graphic art film, a condenser film, a package film and the like.
However, in spite of its excellent intrinsic properties, the film which is molded of only polyethylene terephthalate, is lack of slip property and is inferior in abrasion resistance, so that workability becomes poor in its production processes and in processing therefor.
The slip property and the abrasion resistance of polyester film have a great influence on the film production process and the processing therefor as well as on the quality of an article manufactured therewith. The inferiority of such properties in polyester film would leave scratches on the film during a processing step for the film, which are caused by the friction between a roll and the film, and lead to the generation of a great quantity of white powder.
Particularly, when a magnetic tape that is manufactured by coating a polyester film with a magnetic layer is running in a recording or a playback apparatus in order to record magnetic signals thereon or to playback the magnetic signals, the magnetic tape comes into long contact with the internal parts of the apparatus, such as a head drum and a guide roll. This contact may cause the magnetic tape to become degraded. For example, scratches and white powders are more likely to occur on the surface of the magnetic tape, and these inferiorities are lead to the drop out of the magnetic signals. In addition, repetitive or long time use of the magnetic tape makes the friction coefficient of the magnetic tape increase, deteriorating the traveling ability of the magnetic tape.
In an effort for solving the above-mentioned problems, there has been taken order to reduce the contact areas between the films themselves made of polyester and between the film and the roll by applying to the surface of the film protrusions with the improvement of the slip property and the abrasion resistance of the film.
Prior arts have sought ways of applying the protrusions to the surface of film and thus, improving the film surface properties to a satisfactory level. Prior arts are divided to two kinds.
One of them is a method of forming inner particles that inactivated fine particles are deposited from catalyst residues during the production of polyester.
Another is a method of casting outer particles that inactivated fine particles are added during the production of polyester.
In the method of forming inner particles, there is such an advantage that it is not necessary to pulverize and grade the particles. Moreover, the film formed by the method of forming inner particles relatively has good abrasion resistance since the particles deposited on the film have a good affinity for the polymer and thus, there are little generated the white powders caused by the falling of the particles. This method, however, has difficulty in controlling the diameter of the deposited particles, and there occur differences among the batches which are operated according to the method.
The method of casting outer particles, in which inactivated inorganic particles such as calcium carbonate, calcium sulfate, barium sulfate, kaolin, silica, talc, titanium dioxide, alumina and the like are added when polyester is synthesized or is molded into a film, is advantageous due to the fact that it is possible to freely select the amount and the diameter of particles and stabilize the polymer properties and the shape of particles polymer even when operating for long time. However, in dependence on the kind and the size of the particles, macro granules are formed by the aggregation of the particles, so that, when the film is used as, for example, a base film of magnetic tape, there occur disadvantages, such as drop out and the like. Accordingly, the size, density and shape of particles should be controlled.
In general, the more the particle size of polyester large, the more the slip property of the film made of the polyester is improved. However, the particles with large diameters which are included in polyester film in order to improve the slip property of the film may lower the quality of the final product made of the polyester film, such as a video tape, an audio tape and the like, causing the product used in such a precision field as an electromagnetic field to be deteriorated in electromagnetic transit property. Because the polyester film for high density electromagnetic recording medium is required to have not only superior slip property and abrasion resistance but also excellent surface flatness, the protrusions are preferably fine.